The invention relates to circuit breakers with a magnetic trip unit, and, more particularly, to circuit breakers with a pressure sensitive magnetic trip release mechanism.
Circuit breakers typically provide protection against the very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a magnetic trip unit, which trips the circuit breaker's operating mechanism to open the circuit breaker's main current carrying contacts upon a short circuit condition.
Modern magnetic trip units include a magnet yoke (anvil) disposed about a current carrying strap, an armature (lever) pivotally disposed proximate the anvil, and a spring arranged to bias the armature away from the magnet yoke. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet yoke. The magnetic field acts to rapidly draw the armature towards the magnet yoke, against the bias of the spring. As the armature moves towards the yoke, the end of the armature contacts a trip lever, which is mechanically linked to the circuit breaker operating mechanism. Movement of the trip lever trips the operating mechanism, causing the main current-carrying contacts to open and stop the flow of electrical current to a protected circuit.
In all circuit breakers, the separation of the breaker contacts due to a short circuit causes an electrical arc to form between the separating contacts. The arc causes the formation of relatively high-pressure gases as well as ionization of air molecules within the circuit breaker. These high-pressure gases can cause damage to the circuit breaker casing. The gases, therefore, must be vented from the circuit breaker enclosure. In addition, a phase-to-phase fault can occur if the arc gases from different phases are allowed to mix, and a phase-to-ground fault (e.g. single phase fault) can occur if the gases contact the grounded enclosure. To avoid a phase-to-phase or phase-to-ground fault, gases vented from different phases must be kept separate from each other and away from the grounded enclosure until the ionization has dissipated.
An exhaust port is conventionally employed to vent such gasses in a rotary contact circuit breaker; each phase (pole) employs two pairs of contacts, two contacts of which rotate about a common axis generally perpendicular to the current path from the line side to the load side of the circuit breaker. Each contact set in such an arrangement requires an exhaust port to expel gasses. One of the exhaust ports will be on the line side and one of the exhaust ports will be on the load side of the circuit breaker. In conventional units, the exhaust port on the line side is located proximate the top of the circuit beaker. Since gasses naturally flow in the direction of this port on the line side of the breaker, the port is effective. On the load side of the circuit breaker, the gasses formed consequent to a short circuit naturally migrates toward the lower corner of the breaker. Thus, an exhaust port is located at this corner providing there is sufficient room to exhaust gasses from this port.
An electrical distribution system may contain a series of circuit breakers, namely upstream breakers and downstream breakers. When circuit breakers are connected in series, it is desirable to ensure that a given fault caused by a short circuit condition will trip the circuit breaker closest to the fault. Such selectivity permits downstream breakers connected in series with an upstream breaker to trip without also tripping any upstream breakers. In this way, current to a room in a building can be shut off without shutting off current to the entire building. However, the upstream breaker must also be able to provide adequate protection for the circuit breaker when operating standalone in a non-selective application. If an upstream device trips at too low of a current threshold, there is no selectivity with any downstream breakers. If the upstream device trips at too high of a current threshold, there might not be adequate protection for the circuit breaker or its electrical system. Further, any tripping system must also ensure protection for the circuit breaker and the system in the event of a single-phase condition, e.g. only one phase becomes overloaded. In a multi-phase system, a single-phase condition exists when one pole experiences a fault thereby opening the contacts of that pole. The remaining poles do not experience the fault and therefore their respective contacts remain closed. A single-phase condition is not desirable in an application that uses a multi-phase component such as a three-phase motor. Therefore, it is desirable to provide a circuit breaker tripping system that will trip an upstream circuit breaker at a predefined short circuit fault level while ensuring protection of the circuit breaker and the electrical system should a single phase condition occur and, at the same time, avoiding unnecessary interruption of the performance of the circuit breaker.